Classical fire assaying is and has remained the standard assay for gold and silver in precious metal mining. In this technique, a small rock sample, typically 30-150 grams (gm) is mixed with litharge (lead oxide) and silica fluxes and fused in a high temperature furnace. A resultant lead button containing solubilized gold and silver is poured into a conical mold. This lead button is subsequently cupeled in a bone ash or other cupel whereby the molten lead is converted to lead oxide which is absorbed by the substance of the cupel. When the cupeling is completed, a small dore bead of gold and silver remains.
This bead is parted in nitric acid to remove the silver after recording the initial weight. The parted bead is weighed again giving the gold weight directly. The silver weight is found by difference of the dore bead weight and the gold bead weight. Alternatively, the dore bead can be dissolved in aqua regia and the resultant solution analyzed on an atomic absorption (A.A.) instrument. The original sample may also be digested in aqua regia to solubilize the gold and silver, and the resultant solution analyzed on an A.A. instrument. In this technique a small sample, typically 10 to 15 grams (gm), is digested in the aqua regia.
Some difficulty is encountered with samples that contain very low quantities of gold and silver. Several samples may be fused and the resultant beads added together to obtain greater sensitivity, but this required more time and extra cost. In addition, very great care is required in temperature control and timing to obtain accurate silver assays. While fire assaying is a very sensitive assay technique for gold particularly, which is why it has been used for such a long time, its sensitivity is limited by the size of bead that can be detected and weighed or solubilized in an A.A. finish. There are some organic solvents, such as M.T.B.K., which can extract gold from an aqueous solution and thus concentrate the gold for greater sensitivity on an A.A. instrument.
Additionally, all of these techniques suffer from allowing only small ore samples to be analyzed. In order for the sample data to be meaningful, the small sample must be representative of the larger and typically non homogeneous rock sample from which it is obtained. To date, the only technique which has allowed this representativeness has been a meticulous sample preparation wherein a large rock sample is crushed into successively smaller sizes before being split into a smaller sample. The final sample is pulverized typically to less than 150 mesh prior to be digested or fused for assay. This meticulous sample preparation is not only expensive, but it can be very difficult to achieve with native gold metallics present. Gold is usually not homogeneously distributed in a sample but occurs most commonly as metallics frequently alloyed with silver. While silver can occur as a native metallic, it is more frequently present with gold as electrum or combined with sulfur as a silver sulfide.
The problems are aggravated when the ore deposits are of low grade. The current price of gold and the development of process and mining technology has allowed the development of large low grade deposits. These deposits typically require much drilling and blasting to break the ore for processing. Large volumes of samples are prepared from these blast holes for assaying to control the mining.
The fire assay technique has remained the standard, and low throughput, low sensitivity for low gold content, and the requirement for meticulous sample preparation have been accepted as inevitable.
In view of the above discussion, and object of the present invention is to provide a system which can assay relatively large samples of ore for gold and other assay elements of interest.
An additional object of the invention is to provide an assay system for certain elements in various environments which requires minimal sample preparation.
Yet another object of the invention is to provide an assay system with throughput which is greater than the prior art fire assay method.
Another object of the invention is to provide an assay system with sensitivity sufficient to meet requirements for commercial ore testing and production standards.
Still another object of the invention is to provide an assay system whose accuracy and precision is not adversely affected by typically non homogeneous ore samples and especially relatively large, non homogeneous samples.
Another object of the invention is to provide an assay system that is relatively insensitive to sample geometry.
Yet another object of the invention is to provide a non destructive assay or analysis system that can be used with a wide variety of sample types to determine the elemental concentration of any element susceptible to detection by means of photon activation.
There are other objects and applications of the invention which will become apparent in the following disclosure.